In an image forming apparatus, generally, the surface temperature of a heat roller is detected and the result of the detection is fed back to a heater, thus controlling the temperature of a fixing device. When the heat roller has a thin thickness and a small heat capacity, large temperature fluctuation occurs in the heat roller, depending on fixing conditions, and the temperature of the heat roller fluctuates instantaneously. Therefore, in the conventional technique, when the temperature of the heat roller becomes higher than a predetermined value, wait control of the image forming apparatus is immediately carried out to prevent adverse effects of the temperature rise of the heat roller.
However, when the surface temperature of the heat roller rises to a predetermined value and wait control of the image forming apparatus is suddenly started, there is a risk that the productivity of the image forming apparatus may be significantly lowered.
Thus, it is desired that a temperature controlling method of the fixing device that secures safety and prevents significant reduction in productivity of the image forming apparatus should be developed.